Raman and fluorescent scattering by molecules embedded in dielectric cylinders

Probing dipole and quadrupole resonance mode in non-plasmonic nanowire using Raman spectroscopy

Electric field enhancement in semiconductor nanostructures offers a possibility to find an alternative to the metallic particles which is well known for tuning the light-matter interaction due to its strong polarizability and size-dependent surface plasmon resonance energy. Raman spectroscopy is a powerful technique to monitor the electric field as its scattering depends on the electromagnetic eigenmode of the particle. Here, we observe enhanced polarized Raman scattering from germanium nanowires of different diameters. The incident electromagnetic radiation creates a distribution of the internal electric field inside the naowires which can be enhanced by manipulating the nanowire diameter, the incident electric field and its polarization. Our estimation of the enhancement factor, including its dependence on nanowire diameter, agrees well with the Mie theory for an infinite cylinder. Furthermore, depending on diameter of nanowire and wavelength of incident radiation, polarized Raman study shows dipolar (antenna effect) and quadrupolar resonances, which has never been observed in germanium nanowire. We attempt to understand this polarized Raman behavior using COMSOL Multiphysics simulation, which suggests that the pattern observed is due to photon confinement within the nanowires. Thus, the light scattering direction can be toggled by tuning the polarization of incident excitation and diameter of non plasmonic nanowire.

Diameter and polarization-dependent Raman scattering intensities of semiconductor nanowires

Diameter-dependent Raman scattering in single tapered silicon nanowires is measured and quantitatively reproduced by modeling with finite-difference time-domain simulations. Single crystal tapered silicon nanowires were produced by homoepitaxial radial growth concurrent with vapor-liquid-solid axial growth. Multiple electromagnetic resonances along the nanowire induce broad band light absorption and scattering. Observed Raman scattering intensities for multiple polarization configurations are reproduced by a model that accounts for the internal electromagnetic mode structure of both the exciting and scattered light. Consequences for the application of Stokes to anti-Stokes intensity ratio for the estimation of lattice temperature are discussed.

Raman and fluorescent scattering matrix of spherical microparticles

In this paper, we have investigated the main properties of the Raman and fluorescent matrix of scattering by microspheres using the matrix scattering formalism. The coherent and incoherent inelastic scattering of incident light by a microsphere is described by the Stokes parameters. We demonstrate the main symmetry properties of the coherent and incoherent Raman and fluorescent scattering matrices. Numerical results are presented to illustrate the Raman scattering efficiency, cross-phase coefficient, and some other parameters of scattering by microspheres.

Internal dipole radiation as a tool for particle identification

A numerical approach for the calculation of the internal dipole radiation associated with particles of arbitrary morphology is investigated by using the discrete-dipole approximation (DDA) method. The DDA and analytical solutions for the total radiated power and radiation pattern are compared in the case of spherical host particles. It is shown that the DDA can be quite accurate under the condition that m <or approximately 2, and mkd<0.5, where m is the refractive index of the host particle, k=2pi/lambda is the wavenumber in vacuum, and d is the distance between two adjacent dipoles in the DDA cubic dipole array. Furthermore, the DDA solutions for the dipole radiation patterns associated with nonspherical host particles are compared with their corresponding counterparts obtained from the finite-difference time-domain method. Excellent agreement between the two results is noted. The DDA method is also applied to the computation of the internal dipole radiation associated with simulated nonspherical sporelike particles. The results suggest that the internal dipole radiation patterns contain a great deal of information about the morphology and composition of the host particle.

Enhancement of Raman scattering by deformation of microparticles

Raman scattering on deformed droplets levitated in an acoustic levitator and produced by a vibrating-orifice aerosol generator were investigated. Our samples experiments were diethyl hexyl sebecate (DEHS) droplets in the millimeter-size range and ethanol droplets in the size range 50-100 microm. The C-H stretching region from 2800 to 3100 cm(-1) was investigated. We found that the Raman intensity measured by a scattering angle of 90 degrees depended on the shape of the droplets. Raman scattering on spherical droplets was smaller than scattering on spheroidal droplets with the same volume. Similar results were observed for the fluorescence signal of Rhodamine 6G-doped DEHS droplets.

Electromagnetic fields for a spheroidal particle with an arbitrary embedded source

A spheroidal coordinate separation-of-variables solution has been developed for the determination of the internal, near-surface, and scattered fields of a spheroid (either prolate or oblate) with an embedded source of arbitrary type, location, and orientation. Presented results for (1,0) and (1,1) electric multipoles embedded in 2:1 axis ratio prolate and oblate spheroids (equal volume sphere size parameter equal to 20) illustrate that the presence of the spheroid interface can have a profound effect on the corresponding far-field scattering pattern. The calculation procedure could be used, for example, to model the emission of inelastic scattered light (Raman, fluorescence, etc.) from biological particles of appreciably elongated (prolatelike) or appreciably flattened (oblatelike) geometries.

Geometrical optics calculation of inelastic scattering on large particles

A geometrical optics approximation was used for calculations of inelastic (Raman and fluorescent) scattering on particles with large size parameters. The inelastic part of the radiation was obtained by use of the principle of ray reversibility. The technique presented simplifies the computations and provides a geometric interpretation of how far-field patterns can be calculated by use of the internal field distributions. The numerical results for homogeneous spherical particles are compared with the classic dipole solution.

Angular distribution of fluorescence from dye-filled capillaries

We studied the angular distribution of fluorescence from a small, lossy capillary filled with a laser-dye solution. We found that the fluorescence is isotropic for the liquid core and that, far from the liquid-solid phase transition, this isotropy shows no temperature dependence. This result, an extension of studies with solid cylinders, is at variance with theoretical expectations for solids as well as with previous reports by other investigators but is explained by the motion of the molecules in the liquid. Therefore the optimal viewing angle for capillary zone electrophoresis experiments is near 90° because the elastic scattering of the incident laser light is at or near a minimum for these small capillaries. This reduces contamination of the fluorescence signal as a result of stray laser light in the optical system.

Raman or fluorescent scattering by active molecules or ions embedded in a single-mode optical fiber

By considering active molecules or ions as a collection of induced oscillating dipoles, we treat the problem of Raman or fluorescent scattering by the active molecules or ions embedded in a single-mode optical fiber theoretically. The analytical expressions and the numerical results for the scattering coefficients and the even-odd mode conversion coefficients of the guided modes are given, based on the method of dyadic Green's functions and on the expansions of the modal fields in terms of the vector cylindrical wave functions. We expect to incorporate the treatments into the analysis of a rare-earth-doped fiber amplifier or a fiber Raman amplifier.

Fluorescent angular scattering emissions from dye-filled fibers

Fluorescent angular scattering from laser dye-filled small-core-diameter fibers shows increased backscattered fluorescent emissions as the core diameter decreases below 23 microm. The fluorescent angular scattering from Coumarin 7 laser dye-filled hollow-core quartz fibers were measured at three different fluorescent wavelengths and compared with the elastic scattered incident radiation at 442 nm.

Surface enhanced Raman scattering (SERS) by molecules adsorbed at spherical particles: errata

A model for Raman scattering by a molecule adsorbed at the surface of a spherical particle is articulated by treating the molecule as a classical electric dipole. This follows Moskovits's suggestion [J. Chem. Phys. 69, 4159 (1978)] and the experiments by Creighton et al. [J. Chem. Soc. Faraday Trans. II, 75, 790 (1979)] that such a system may exhibit SERS similar to that at roughened electrode surfaces. The molecule is stimulated by a primary field comprised of the incident and near-scattered fields. Emission consists of the dipole field plus a scattered field, each at the shifted frequency. Addition of feedback terms between the dipole and the particle makes only a negligible contribution to the fields. For pyridine adsorbed at the surface of a silver sphere, the 1010-cm(-1) band is enhanced by ~10(6) if the radius is much less than the wavelengths and the excitation wavelength is ~382 nm, a wavelength for which the relative refractive index of silver is close to m = radical2i. Detailed results are given for the effect upon the angular distribution and the polarization of the Raman emission of particle size, distance from the surface, excitation wavelength, and location of the molecule upon the surface. These results simulate those observed at roughened silver electrodes and suggest that the mechanism of SERS at those electrodes may resemble the electromagnetic mechanism elucidated here. We predict that comparable effects should be observed for fluorescent scattering.

Surface enhanced Raman scattering (SERS) by molecules adsorbed at spherical particles

A model for Raman scattering by a molecule adsorbed at the surface of a spherical particle is articulated by treating the molecule as a classical electric dipole. This follows Moskovits's suggestion [J. Chem. Phys. 69, 4159 (1978)] and the experiments by Creighton et al. [J. Chem. Soc. Faraday Trans. II, 75, 790 (1979)] that such a system may exhibit SERS similar to that at roughened electrode surfaces. The molecule is stimulated by a primary field comprised of the incident and near-scattered fields. Emission consists of the dipole field plus a scattered field, each at the shifted frequency. Addition of feedback terms between the dipole and the particle makes only a negligible contribution to the fields. For pyridine adsorbed at the surface of a silver sphere, the 1010-cm(-1) band is enhanced by ~10(6) if the radius is much less than the wavelengths and the excitation wavelength is ~382 nm, a wavelength for which the relative refractive index of silver is close to m = radical2i. Detailed results are given for the effect, upon the angular distribution and the polarization of the Raman emission, of particle size, distance from the surface, excitation wavelength, and location of the molecule upon the surface. These results simulate those observed at roughened silver electrodes and suggest that the mechanism of SERS at those electrodes may resemble the electromagnetic mechanism elucidated here. We predict that comparable effects should be observed for fluorescent scattering.

Raman and fluorescent scattering by molecules embedded in dielectric spheroids

Fluorescent and Raman scattering by molecules embedded in dielectric particles is strongly dependent on the morphology and optical properties of the particle, the distribution of active molecules within the particle, and, in the case of nonspherical particles, orientation. The model previously applied to spheres and cylinders is now extended to spheroids. The extended boundary condition method (EBCM) has been used to calculate the transmitted field at the incident frequency that stimulates the process. The equivalence principle underlying the EBCM has also been applied to calculate the fields at the shifted frequency. Numerical results are presented to illustrate some of the effects of refractive index, size, shape, and orientation of the particles for models representing two polarizabilities of active dipoles embedded inside the particles.